Greenwald writes this act of intimidation “will only embolden us more to continue to report aggressively.”

The partner of the Guardian journalist who has written a series of stories revealing mass surveillance programmes by the US National Security Agency was held for almost nine hours on Sunday by UK authorities as he passed through London‘s Heathrow airport on his way home to Rio de Janeiro.

David Miranda, who lives with Glenn Greenwald, was returning from a trip to Berlin when he was stopped by officers at 8.30am and informed that he was to be questioned under schedule 7 of the Terrorism Act 2000. The controversial law, which applies only at airports, ports and border areas, allows officers to stop, search, question and detain individuals.

Miranda was then released without charge, but officials confiscated electronics equipment including his mobile phone, laptop, camera, memory sticks, DVDs and games consoles.

Since 5 June, Greenwald has written a series of stories revealing the NSA‘s electronic surveillance programmes, detailed in thousands of files passed to him by whistleblower Edward Snowden. The Guardian has also published a number of stories about blanket electronic surveillance by Britain’s GCHQ, also based on documents from Snowden.

While in Berlin, Miranda had visited Laura Poitras, the US film-maker who has also been working on the Snowden files with Greenwald and the Guardian.

“This is a profound attack on press freedoms and the news gathering process,” said Greenwald. “To detain my partner for a full nine hours while denying him a lawyer, and then seize large amounts of his possessions, is clearly intended to send a message of intimidation to those of us who have been reporting on the NSA and GCHQ. The actions of the UK pose a serious threat to journalists everywhere.

“But the last thing it will do is intimidate or deter us in any way from doing our job as journalists. Quite the contrary: it will only embolden us more to continue to report aggressively.”

A spokesperson for the Guardian said: “We were dismayed that the partner of a Guardian journalist who has been writing about the security services was detained for nearly nine hours while passing through Heathrow airport. We are urgently seeking clarification from the British authorities.”

A spokesperson for Scotland Yard said: “At 08:05 on Sunday 18 August 2013 a 28-year-old man was detained at Heathrow airport under schedule 7 of the Terrorism Act 2000. He was not arrested. He was subsequently released at 17:00.”

Schedule 7 of the Terrorism Act has been widely criticised for giving police broad powers under the guise of anti-terror legislation to stop and search individuals without prior authorisation or reasonable suspicion – setting it apart from other police powers. Those stopped have no automatic right to legal advice and it is a criminal offense to refuse to cooperate with questioning under schedule 7, which critics say is a curtailment of the right to silence.

Last month, the UK government announced it would reduce the maximum period of detention to six hours, and promised a review of the operation on schedule 7 amid concerns that it unfairly targets minority groups and gives individuals fewer legal protections than they would have if detained at a police station.